Liquid crystal devices

ABSTRACT

This invention relates to liquid crystal display devices. One aspect of the invention concerns the electrolytic growing of projections on one plate of the device so that the projections register with the tin-oxide segment array on the other plate; another aspect concerns utilizing similar projections as internally placed spacers. An insulative sheet is deposited over the segment array in order to filter D.C. components, according to a third aspect of the invention, and, according to a fourth aspect, a spacing technique is disclosed which utilizes alumina particles in a glass matrix as an edge spacer. Further, a light concentration technique, utilizing a one-way mirror and a prism or fiber optic assembly, is disclosed.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation, of application Ser. No. 272,524 filed July 17,1972, now abandoned.

The convention priority application for this invention is Italianapplication No. 70842-A/71 filed on Nov. 24, 1971, in Italy.

BACKGROUND OF THE INVENTION

Liquid crystals have recently found wide application in the display artdue to their ability to switch from a transparent to an opaque mode inthe presence of electric fields. However, the prior art devices (whichwill be discussed in detail hereinafter) are subject to a number ofsubstantial disadvantages, among them, poor definition, short life andproblems of assembly.

OBJECTS OF THE INVENTION

It is the primary object of this invention to overcome the disadvantagesof the prior art liquid crystal devices.

It is a further object of this invention to increase the visualdefinition of displayed characters.

It is a further object of this invention to provide a display of longerlife than currently available.

It is a further object of this invention to provide simple and effectmeans for precisely spacing the glass plates which are used in liquidcrystal displays.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1a shows a conventional liquid crystal display used in thetransmissive mode; FIG. 1b shows one plate of a conventional liquidcrystal display.

FIG. 2 shows a conventional liquid crystal display used in thereflective mode.

FIG. 3 shows one plate of a display utilizing projections which registerwith the segment array of the plate opposite.

FIG. 4 shows the segment array covered with a film of insulativematerial;

FIG. 5 shows a side view of a display utilizing alumina-glass as an edgespacer.

FIG. 6 a and b, shows both plates of a display constructed with internalspacers.

FIG. 7a shows a transmissive display adapted to utilize ambient light;FIG. 7b shows a bundle of optical fibers which can be used in thestructure of FIG. 7a.

DETAILED DESCRIPTION

FIGS. 1a and b show a typical state of the art liquid crystal displaydevice consisting of two parallel glass plates 1 and 2 having a layer ofliquid crystal material 3 sandwiched between. Transparent conductivecoatings 4 and 5, which are generally of tin oxide, are deposited overthe inner surfaces of the glass plates; on one plate, the conductivecoating is applied so as to form the familiar seven segment arrangementas seen in FIG. 1b; on the other plate the coating is applied so as tocompletely cover the enclosed surface of the glass plate. Electrodes 6(FIG. 1b) are deposited along with the array 4 and both of theconductive, transparent coatings 4 and 5 are connected to a source ofvoltage. Polymeric spacers 7 are bonded to the plates with epoxy resinto complete the fabrication.

FIG. 1a shows a liquid crystal display designed or used in thetransmissive mode. A ray of light 10 is directed at the side of thedisplay which is opposite the observer 9. Since the glass plates andconductive coatings are transparent, the observer sees light ray 11which is passed unaltered through the display; the observer sees,therefore, a uniformly bright upper surface. When an electric field isimpressed across the array 4 and the sheet 5, the liquid crystal in thepath of the field becomes turbulent, causing the incident light ray 10to be scattered. The scattered light 12 does not reach the observer and,consequently, he sees a rather milky pattern on a light field, whichpattern corresponds to the particular segments of array 4 which wereenergized.

FIG. 2 shows the typical liquid crystal display used in the reflectivemode. A reflective sheet 8 is applied to the outer surface of the glassplate 2 which is opposite the observer 9. A ray of incident light 10 iscaused to be directed toward the surface of upper plate 1; since theliquid crystal is transparent the light ray 10 is reflected in thenormal manner (angle of incidence equaling the angle of reflection) andno light reaches the eye of the observer 9. When a voltage is impressedacross the conductive sheet 5 and conductive array 4, the liquid crystalwhich is subjected to the consequent electric field loses itstransparency and causes the incident light ray 10 to be scattered. Aportion of the scattered light 12 is directed back to the eye of theobserver 9 and he sees a light pattern on a dark field, which patterncorresponds to the particular segments of array 4 which have beenenergized.

FIG. 3 shows one of the glass plates of the liquid crystal displayconstructed in accordance with one aspect of the invention. The other(opposite) plate which contains the aforedescribed seven segment arrayis not shown. Glass plate 20 is coated with a thin sheet of reflectivemetal (e.g. aluminum or silver) 21, which sheet is connected to analternating voltage source A.C. Sheet 21 carries projections 22 whichare placed so as to register with the transparent tin oxide sevensegment array carried by the upper plate. The metallic sheet serves twopurposes: to create, along with the tin oxide array, the electric fieldthrough the liquid crystal and to reflect light rays which are incidentfrom above. The plate of FIG. 3 will therefore be used only in thereflective displays generally described in connection with FIG. 2. (Atransmissive display can be constructed by substituting tin-oxide forthe reflective metal.)

Projections 22 are made by conventional electrolytic growing; that is,sheet 21 is masked with an insulative material over all areas exceptthose where the projections are desired, the plate is then placed in anelectrolytic bath (e.g. aluminum or silver), made a cathode, and theprojections are grown. By monitoring the total charge transfer, thethickness of the projections can be very accurately controlled.Typically the projections will be 15 microns in thickness with thedistance between the upper plate and sheet 21 being 25 microns, therebymaking the distance between the upper plate and upper surface of theprojections 22 to be 10 microns.

The physics of liquid crystal phenomenon is not entirely understood butit is known that the transparent-opaque switching property is caused bya mechanical resonance of the crystal molecules, which resonance dependson both the thickness of the liquid crystal and the frequency of theexcitation voltage. It has been found that, for a given thickness, thereis a particular frequency value above which no switching occurs and thatas the thickness decreases, the frequency value increases.

The exact frequency above which no switching occurs is highly dependenton the particular liquid crystal utilized and can range from 50 to 5000c/s. In any event, for the embodiment described above, the cut-offfrequency between the upper plate and the projections will be 10 timesas great as the cut-off frequency between the upper plate and theunraised areas of the lower plate.

By utilizing the grown projections 22, the turbulence in the liquidcrystal can be precisely limited to that crystal which is between theprojections and the tin-oxide array. The displayed numbers areconsequently more sharply defined than those of a conventional display;in addition, there will be no superfluous glows caused by the electricfields which emanate from the leads 6 connected to the tin-oxide array 4(FIG. 1b). The glows caused by these leads 6 are quite distracting; inorder to limit their effect, the prior art devices require that eachdisplay unit (FIG. 1b) be surrounded by a window frame mask which hidesthe more substantial glows which occur about the edges of each unit.These masks cause a multi-character display to appear as a row ofwindows, each character being segregated from those adjoining by darkstrips of mask. The effect is unsightly and further causes the observerto tend to read individual digits rather than the single multi-digitnumber. The utilization of the disclosed projections eliminates thesemasks.

FIG. 4 shows the tin-oxide seven segment array 4 deposited on glassplate 1 in the conventional fashion. However, deposited over the plate 1and array 4 is a thin layer of insulative material 30 (e.g. Si O₂).(This insulative layer, while shown covering the entire surface of theplate 1, need only cover the segments of the array 4). The insulativelayer 30 causes the display to behave as a capacitor; any D.C. currentswhich are impressed across the liquid crystal gap are blocked, whileA.C. current is conducted. The insulative layer 30 filters all D.C.components; since it is the presence of D.C. current components whichcause failure in liquid crystal displays, the utilization of theinsulative layer 30 enables one to obtain a display which will have alonger life than the conventional devices.

One of the more difficult problems which are encountered in theconstruction of liquid crystal display is assuring that the glass platesare permanently spaced at a proper distance. The conventional technique,as described in connection with FIG. 1a, is to use polymeric spacers 7which are affixed to the glass plates with epoxy resin. This techniquehas a number of rather serious drawbacks, one being that the organicliquid crystal tends to dissolve the organic spacer and resin.

One solution to the problem is depicted in FIG. 5. FIG. 5 is a highlysimplified representation of a liquid crystal display; only glass plates1 and 2 are shown but it is to be understood that the seven segmentarray and other elements necessary to a liquid crystal device areproperly affixed. About the edges of a glass plate is deposited amixture of powdered glass and alumina which is conveniently applied byconventional silk screen techniques. Powdered alumina, which consists ofgrains of highly controlled diameter, is currently commerciallyavailable. Alumina grains of 8 to 10 microns in diameter are mixed withpowdered glass, the mixture is deposited about the edges of one of theglass plates, the other plate is pressed over the lower plate with themixture sandwiched between, and the assembly is heated to the meltingpoint of the glass. The alumina particles (which have a higher meltingpoint than glass) form a spacer ridge while the glass, uponresolidification, securely bonds the assembly. FIG. 5 shows aluminagrains 31 fixed in glass matrix 32. Should there be a relatively smallnumber of alumina grains which are of overly large diameter, thesegrains will penetrate the surface of the glass plates since alumina isfar harder than glass. The plates will therefore be separated by aproper distance, even in the presence of out-of-tolerance alumina grainsin the mixture.

FIG. 6, a and b, depict the upper and lower plates of a liquid crystaldisplay constructed in accordance with another aspect of the invention.The problem of precisely spacing the plates of a liquid crystal deviceis not completely solved by assuring that the plates are properlyseparated along their edges - one must also be certain that the centralregions of the display are separated by the precise distance required.FIG. 6a shows the upper plate having the seven segment array and furtherhaving two deposited conductive regions 51 and 52 with end tabs 53 and54. These regions and tabs can be of tin-oxide or metal and, iftin-oxide, can be deposited simultaneously with the seven segmentarrays. The bottom plate seen in FIG. 6b has two projections 55 and 56which are electrolytically grown on the metallic sheet 57; theseprojections are grown in the same fashion as those described inconnection with FIG. 3. Projections 55 and 56, however, are grown to athickness which is equal to the desired spacing between the plates. Tabs53 and 54 and projections 55 and 56 are so located so that when theplates are placed together, the tabs and projections will make contact.During the assembly process, after the plates are placed together, asmall voltage V is connected to the metallic plate 57 while conductiveregions 51 and 52 are monitored. If the voltage is conducted to regions51 and 52, it is known that plates are properly spaced in the internalregions. Should the voltage fail to appear at 51 or 52, it is known thatsome error has been made and the display must be reassembled ordiscarded.

The tabs and regions 51-54 are utilized only during the assemblyprocess; once it has been determined that the plates are properlyspaced, the tabs and regions are ignored. Projections 55 and 56,however, continue to serve the function of keeping the plates properlyspaced; the projections assure that the plates will not be bent towardeach other throughout the life of the display.

(A modification of FIG. 6b would be to electrically isolate eachprojection from sheet 57 by etching paths through the sheet down to theglass plate. Each path would almost surround each projection and wouldextend to the edge of the plate to form a "key hole" shapedconfiguration. Each projection together with a thin channel of sheet 57would be electrically isolated from the rest of the sheet. Duringdisplay operation, the projections would therefore be unenergized andthere would be no danger of superfluous glows. Another modificationwould be to impress the test voltage on one of the conductive regions(e.g. 51) and monitor the other region 52; by so doing, one would nothave to impress any test voltages on the lower plate.)

As seen in connection with FIG. 1a, liquid crystal displays can be usedin the transmissive mode, that is, with light passing completely throughthe display before reaching the observer. When used in the transmissivemode, however, it is necessary to place an artificial light sourcebehind the display in order to obtain adequate light passage. FIG. 7ashows a technique whereby the liquid crystal display 60 can be used inthe transmissive mode using only ambient light. Placed behind thedisplay is a prism 61 (which can be a conventional rectangular type)positioned so that light passing through the prism is concentrated aboutthe seven segment array (not shown) of the display 60. Instead of prism61, a bundle of optical fibers 70 can be used (see FIG. 7b).

Display 60 is usually read at an angle of about 45°, that is, theconventional, comfortable angle at which an observer 62 would read anydocument. At this angle, the back of the display is directed at thelower portions of the room where the ambient light is weakest. Byplacing prism 61 behind the display, the stronger ambient light from theupper portions of the room is utilized; furthermore, the prism can beselected to give any desired degree of light concentration.

In addition to prism 61 or optical fibers 70 (FIG. 7b), a "one-waymirror" 63 is placed behind the display. The one-way mirror 63 is ofconventional construction (that is, a vacuum deposition of metal orsequence of insulators on glass). Mirror 63 allows the prismconcentrated light to pass to the display and, further, reflects theambient light 64 through the display. The reflected ambient light 64 isrelatively intense since it emanates from the upper portions of theroom.

While the various inventive concepts disclosed have been described inconnection with numerical display devices, it must be clear that theinvention is not to be limited to these particular devices; theinventions can obviously be used in many liquid crystal applications.

In order that they be more easily understood, the various aspects of theinvention were described separately; however, it is clear that two ormore of these aspects can be incorporated in a single device, dependingupon the designers' specific requirements.

What is claimed is:
 1. In a device having two parallel members adaptedto contain liquid crystal material therebetween and spacer means forspacing said members from each other a given distance and attaching sameto each other, wherein the improvement comprises:said spacer meanscomprising alumina particles in a glass matrix and disposed proximate tothe edges of said members with the diameter of the particles equal tosaid given distance and wherein the glass matrix is in a molten stateduring the assembly of said device and solidifies attaching said membersto each other with said members spaced apart said given distance by saidparticles.
 2. In a device according to claim 1, wherein said spacingmeans further comprisesa conductive projection centrally situated on thefirst of said members and having a thickness equal to said giveninstance, and a conductive area centrally situated on said second memberand contacting said projection, whereby the central area of the twomembers are spaced apart an equal distance to that of the edges and thepassage of a voltage from said conductive projection to said conductivearea during the assembly assures that said conductive projectioncontacts said conductive area.